The present invention relates generally to x-ray tubes. More particularly, the present invention relates to systems and methods for balancing mechanical loads in x-ray tubes.
X-ray tubes have found widespread application in devices such as imaging systems. X-ray imaging systems utilize an x-ray tube to emit an x-ray beam which is directed toward an object to be imaged. The x-ray beam and the interposed object interact to produce a response that is received by one or more detectors. The imaging system then processes the detected response signals to generate an image of the object.
For example, in computed tomography (CT) imaging, an x-ray tube projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third-generation CT systems, the x-ray tube and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e. projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object.
Typically, an x-ray tube comprises a vacuum vessel, a cathode assembly, and an anode assembly. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. The cathode assembly and the anode assembly are enclosed within the vacuum vessel.
To generate an x-ray beam, the cathode is heated to a temperature at which the cathode begins to emit electrons. A voltage difference (typically, in the range of 60 kV to 140 kV) is maintained between the cathode and anode assemblies and accelerates the electrons, causing the electrons to impact a target zone of the anode at high velocity. Upon impact, a small fraction (less than 1%) of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance produces heat. The x-rays emanate from a focal spot of the target zone in all directions, and a collimator is then used to direct x-rays out of the vacuum vessel in the form of an x-ray beam toward the patient.
In the first x-ray tube designs, the anode assembly remained stationary. However, due to the large amount of heat that is produced (the focal spot of the anode can reach temperatures of about 2700xc2x0 C.), a rotating anode design has been adopted for many applications. According to this design, the anode assembly includes a rotating disk and the focal spot moves along a target track on the anode. This prevents material on the anode from melting, in a manner generally analogous to the manner in which waiving one""s hand over a candle rather than holding one""s hand directly over the candle prevents one""s hand from burning.
Although the rotating anode design is advantageous in that it promotes heat dissipation, the rotating anode design provides additional challenges inasmuch as two rotating systems are employed. Specifically, the x-ray tube comprises a rotating anode assembly that rotates within the x-ray tube about a tube axis of rotation, and the x-ray tube itself is mounted to a gantry which is rotating about a gantry axis of rotation (e.g., which may be aligned with a patient).
A difficulty that has been encountered is uneven loading of bearings that support the rotating anode assembly. Rotating anode assemblies have used a cantilevered design in which the rotating disk is mounted at one end of a rotating shaft, with the other end of the rotating shaft being supported by two or more bearing assemblies. As the x-ray tube rotates about the gantry axis of rotation, the resultant centrifugal force that is applied to the x-ray tube is opposed primarily by the bearing assembly that is closer to the rotating disk (closer to the center of gravity), resulting in uneven loading. This is undesirable because it causes premature failure of the bearing assemblies, especially the bearing assembly that provides primary opposition to the centrifugal force caused by rotation of the gantry.
In order to improve performance characteristics of CT systems, it is desirable to increase the gantry rotational speeds that are employed. Increased speeds, however, increase the bearing loads since centrifugal force is proportional to the square of the gantry rotational speed. Therefore, the inability to obtain increased gantry speeds without premature bearing failure has become a limiting factor in the development of CT systems.
Therefore, an improved x-ray tube and method of balancing mechanical loads in an x-ray tube would be highly advantageous.
In a first preferred aspect of the invention, a computed tomography system comprises a gantry and an x-ray tube. The gantry rotates about a gantry axis of rotation. The x-ray tube is mounted to the gantry, and comprises a rotatable assembly having a tube axis of rotation. The tube axis of rotation is angularly displaced from the gantry axis of rotation by a tilt angle. Rotation of the x-ray tube about the gantry axis of rotation produces a centrifugal force that is applied to the x-ray tube. Rotation of the rotatable assembly about the tube axis of rotation produces a gyroscopic moment that results in an additional force being applied to the rotatable assembly that opposes the centrifugal force.
In a second preferred aspect, a method of operating a computed tomography system comprises producing a first moment that acts upon an x-ray tube, and producing a second moment that acts upon the x-ray tube while the first moment is being produced. The x-ray tube being mounted to the gantry. The first moment is produced by rotation of a gantry about a gantry axis of rotation at a gantry rotational speed. The second moment is produced by rotation of a rotating assembly of the x-ray tube about a tube axis of rotation. The tube axis of rotation is tilted with respect to the gantry axis of rotation. The second moment is a gyroscopic moment that is produced by precession of the rotatable assembly. The precession occurs by way of the rotation of the x-ray tube about the gantry axis of rotation and the rotation of the rotatable assembly about the tube axis of rotation. The tube axis of rotation of the rotatable assembly defines an outer surface of a portion of a cone as the rotatable assembly rotates about the gantry axis of rotation.
In a third preferred aspect, a computed tomography system comprises a gantry and an x-ray tube. The gantry rotates about a gantry axis of rotation. The x-ray tube is mounted to the gantry, and comprises a rotatable assembly having a tube axis of rotation. The tube axis of rotation is angularly displaced from the gantry axis of rotation by a tilt angle. Rotation of the x-ray tube about the gantry axis of rotation produces a first moment, and rotation of the rotatable assembly about the tube axis produces a second moment that opposes the first moment.
Advantageously, in the preferred embodiments, the tilt angle causes a gyroscopic moment to be produced which can be used to balance loading in the x-ray tube. Therefore, although it has long been assumed that the tube axis of rotation and the gantry axis of rotation must be parallel, it has surprisingly been found that this is not necessarily the case and that in fact introducing a tilt angle can have significant benefits.
Other principle features and advantages of the present invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.